1. Field of the Invention
The prevent invention relates to an alkaline storage cell including a positive electrode mainly comprising nickel hydroxide, such as a nickel-cadmium cell and a nickel-hydrogen cell.
2. Description of the Prior Art
A well-known method of preventing the nickel electrode plate of an alkaline storage cell from expanding with the progress of the charge/discharge cycle test is adding cadmium to the nickel electrode. It is also known that adding cobalt together with cadmium improves the chargeability in a high temperature and the utilization factor of the active material and also suppresses self-discharging as well as preventing the electrode plate expansion. The followings are some of the publications disclosing the above ideas, and effects thereof.
1. Japanese Patent Publication No. 42-21115: suppression of electrode plate expansion and improvement of chargeability in a high temperature
2. Japanese Patent Publication No. 60-12742: suppression of electrode plate expansion and improvement of active material utilization factor
3. Japanese Patent Publication No. 59-10538: improvement of chargeability in a high temperature and restriction of self-discharging
4. Japanese Patent Publication Laid-Open No. 48-53228: improvement of chargeability in a high temperature
5. Japanese Patent Publication Laid-Open No. 51-87733: improvement of chargeability in a high temperature
6. Japanese Patent Publication Laid-Open No. 52-114928: improvement of chargeability in a high temperature
7. Japanese Patent Publication Laid-Open No. 52-114929: improvement of chargeability in a high temperature
8. Japanese Patent Publication Laid-Open No. 62-86661: suppression of electrode plate expansion
However, the use of cadmium is now more and more restricted for environmental protection. To cope with such a trend, impregnating the nickel plaque with an active material added with zinc or a zinc compound has recently been proposed (for example, Japanese Patent Publication Kokai No. 59-83347 and D. H. Fritts "Zinc Hydroxide as a Substitute for Cobalt Hydroxide in Nickel Electrodes", The Electrochemical Society Inc. 160th Meeting Extended Abstracts, Vol. 81-2,page 86 (1981)). However, simply adding zinc or a zinc compound to the active material is not enough to suppress the electrode plate expansion.
In consideration of all the above, it has been proposed to deposit the active material and zinc (or the zinc compound) together to form a solid solution (refer to Speeches for the 29th Cell Symposium, 1988, page 53). However, a cell including a nickel electrode which has the above solid solution is extremely low in chargeability, especially in a high charging temperature where oxygen overvoltage is lowered to promote the oxygen gas generation.
This problem is solved by adding a cobalt compound to the active material of the nickel electrode. Such a method can improve the chargeability of the active material but cannot suppress the electrode plate expansion sufficiently.
Another conceivable method is adding cobalt and zinc to the nickel electrode. This method requires a huge amount of cobalt in order to improve the chargeability against the adverse effect of zinc of lowering the chargeability. Accordingly, the impregnating amount of the active material is reduced, with the result of a sharp drop of the cell capacity.